1. Technical Field
The present invention relates to a power generating device and an electronic device.
2. Related Art
There is a power generating device which has a fixed substrate and a movable substrate arranged opposite to each other and effects power generation of an electrostatic induction type using vibration of the movable substrate.
In this case, for example, the fixed substrate includes, on a surface opposing the movable substrate, a fixed electrode having a conductor and a charging member (electret) overlapping each other and processed into a comb shape.
The movable substrate includes a movable electrode overlapping part of the fixed electrode in plan view of the fixed substrate on a surface opposing the fixed electrode of the fixed substrate.
When the movable substrate vibrates in the planar direction of the fixed substrate, vibration energy applied to the movable electrode changes the Coulomb attraction between the fixed electrode and the movable electrode. The change is collected as power to be converted into electric energy. That is, power generation is carried out.
When the movable substrate does not return to a static position, the generation efficiency drops. Therefore, it is preferable that the movable substrate return to a static position after the vibration. The term “a static position” indicates “a stop position,” at which the movable substrate stops. Electrostatic energy generated between the fixed substrate and the movable substrate becomes a minimum when the movable substrate stops at the position. Various techniques have been proposed to return the movable substrate to the static position.
As shown in FIG. 11, for example, a second substrate (fixed substrate) 405 having a plurality of electret material regions 409 and a first substrate (movable substrate) 407 having a plurality of conductive surface regions 411 are arranged at a predetermined distance therebetween in JP-T-2005-529574.
In this case, the second substrate (fixed substrate) 405 including the electret material regions 409 is fixed, and the first substrate (movable substrate) 407 including the conductive surface regions 411 is coupled to fixed structures 417 via support springs 419. JP-T-2005-529574 discloses that the support springs 419 are connected to both side surfaces of the first substrate 407, and cause the first substrate 407 to make motion in the X direction (direction indicated by an arrow 421) and return to a static position.
JP-A-2008-113517 discloses that when magnetic bearings are used for movable portions 4 and magnets 3 are embedded in a fixed substrate 1, as shown in FIG. 10, the movable portions 4 can be returned to respective static positions by attraction generated by a magnetic field.
When the structure using the support springs 419 to return the movable substrate to the static position is employed as described in JP-T-2005-529574, however, the springs repeat compression and expansion movements during power generation. Therefore, the springs are likely to fatigue and deteriorate, which may reduce the reliability. When power generation is carried out at 50 Hz for one year, for example, the springs repeat compression and expansion movements about 1.6 billion times in total. Depending on the use conditions, the life of ordinary springs is 10 million to 100 million times or so in terms of the number of compression and expansion movements, which makes it difficult to maintain the reliability of the springs over a long period of time.
When springs are used, it is possible to convert energy of vibration in one axial direction to effect power generation, but it is difficult to convert energy of vibration in two axial directions (planar direction) to effect power generation because the springs supporting the movable substrate are twisted. This makes it difficult to improve the efficiency of converting vibration energy.
As described in JP-A-2008-113517, when the structure in which the movable substrate is returned to the static position by the magnets 3 is employed, the magnets 3 which are not actually used in power generation of the electrostatic induction type are included as components. Accordingly, it is necessary to design a manufacturing process that is not affected by the magnets 3. This increases the number of steps in the manufacturing process and may cause the production line to be damaged by an unexpected influence.
While strong attraction is generated near the magnets 3, the attraction becomes weaker when the magnets 3 and the bearing move away from each other. If the attraction becomes weak, the vibration of the movable substrate becomes small. Accordingly, the generation efficiency drops. When strong vibrations are applied to the movable substrate, it moves away from the magnets 3. The magnetic force becomes smaller as the distance to the magnets 3 becomes greater. When large vibration energy is applied, therefore, it takes time to return the movable substrate to the static position from a position far from the static position. In other words, the number of power generations per unit time decreases, so that the generation efficiency drops.